Formerly, computer networks were in the province of academia, large research communities, corporations, and governmental agencies. Over time, these networks have evolved to contain many more computer users and uses within their domains. Furthermore, the networks have to a large extent become connected ("internetworked") to form a world wide network of computers known as the Internet and new types of computer networks have become available. Thus, it has become more likely than not that a computer is connected to some type of computer network.
A consequence of this ever increasing connectivity of computers is that the amount of information available "on-line" has become huge. For example, as of August, 1995, one popular search tool of information available on the Internet (Lycos, http:.backslash..backslash.www.lycos.com) contained an index of 5.6 million web-pages. Similarly, the on-line services, such as America Online, Compuserve, and Prodigy, compete based on the amount of information available through their particular services.
While information distribution is an important use of computer networks, computer connectivity provides many other valuable services. One example is electronic mail. Another is electronic commerce.
For all these reasons, it has become generally recognized that while in the past it was true that being connected to a network or an on-line service could provide a competitive advantage, in modern business and for the sake of education, it has become a necessity.
Many users of on-line services and the Internet connect to these networks via modems and conventional telephone lines. While this is adequate for some uses, it is undesirable for others. It has the advantage of providing a dedicated connection between amuser and a computer to which that user is connected. And it makes use of a connection line, the telephone line, which most households and offices are already connected to. By providing a dedicated connection to the user, there is no need for resolving collisions between users who attempt to access the network simultaneously. All such conflicts are resolved at the computer to which the multiple users are connected. (Of course, if that computer is connected to other computers on shared lines, conflicts between these computers must be resolved).
However, telephone lines are a less than ideal way of connecting computers. First, there is the obvious drawback that in a home-computing environment, a home may be equipped with only one telephone line. Thus, extended on-line use would tie-up the telephone line, which might be needed for other uses. There are also technical problems with telephone lines. These lines are intended for voice communication having a transmission band-width limited to 3.2 KHz. As a consequence, digital communication over telephone lines is of a relatively low bit rate. Typically, a modern modem connecting a personal computer to a telephone line is limited to 28.8 kbps.
It is therefore desirable to provide alternative technologies for connecting home computers to computer networks.
One type of technology that increases the band-width for connections of home computers and small-office computers to computer networks is ISDN lines. These are dedicated lines intended for digital communication between computers. ISDN has much higher band-width than telephone lines. However, most homes and small-offices are not equipped with ISDN lines and few are prepared to take on the expense of having ISDN installed.
A technology that has made its way to most homes and small-offices is cable tv networks. Unlike telephone networks, which provide dedicated lines to each home, in cable tv systems all subscribers in a neighborhood share the cable. However, there is much greater band-width available, as compared to telephone lines, on the cables used by most cable tv systems.
For these reasons it is desirable to provide two-way data communication between home/small-office computers and computer networks over existing cable tv systems.
Because all the cable tv subscribers in one neighborhood share one cable, there is a possibility that multiple computers connected to such a network would simultaneously transmit data on that cable. Such concurrent transmission would result in garbled data and neither transmitter's message would be successfully received by the intended recipient, unless there is a mechanism for avoiding or resolving such contention problems.
The problem of avoiding contentions on shared computer networks have been addressed in a variety of ways. One early approach is known as the ALOHA system. ALOHA originated at the University of Hawaii and was initially primarily used for communication using broadcast radio. The communication over a shared cable is also a broadcasting system in that all stations attached to the cable receive any signal that is transmitted over that cable.
ALOHA's basic principle is to allow every user who shares the common channel to broadcast a message at any given point in time. The transmitting user then listens to the channel for its own message. If the message is not received (by the user who sent it), the user knows that the message must for some reason have become destroyed. Very likely, the message was destroyed by another message simultaneously sent by another user. To resolve this contention for the common channel both users "back-off" a random amount of time before resending the message. Thus, if two users attempt to send a message at time t, their respective messages would become destroyed. By backing off random amounts of time, the first user would resend its message at t+r1 and the second user its message at t+r2. If r1 and r2 are different (which they should usually be if they are truly randomly selected), there should be no conflict between the two messages the second time they are sent.
A drawback to ALOHA is that the likelihood of contention is very high. The channel efficiency rate for ALOHA is a mere 18%.
One improvement of ALOHA is slotted-ALOHA. In slotted-ALOHA, time is divided into specified intervals and messages may only be sent during these intervals, i.e., no message is allowed to span over interval boundaries. The channel efficiency of slotted-ALOHA is 38%.
For a discussion of ALOHA, slotted-ALOHA and the derivation of the channel efficiency rates see, Tanenbaum, Andrew, Computer Networks, second edition, Prentice-Hall, pp. 121-124.
Current solutions for data communication on Cable TV systems include LANMCN-AT of Zenith Corporation and LCP of LANcity Corporation, Andover, Mass. These products share a common design philosophy. In both, the upstream data, i.e., the data transmitted from the end-user to the cable system head-end, is transmitted through the lower band of the broadcast spectrum (below the frequency bands dedicated for downstream data). This upstream data is manipulated in one of two ways. The first alternative is to frequency translate to a higher band of the spectrum, and the second is to demodulate the data. The manipulated data is then re-transmitted on the high-band. All modems, including the modem at the head-end site, transmit on the low band. The transmitted data is either frequency translated or demodulated. All modems, then, receive on the high band.
The upstream data and downstream data have the same bandwidth and data rate. In fact, because the modem at the head-end is treated like all other modems in the system, there is no distinction between upstream and downstream data.
There are several problems with this type of solution for digital data communication on a cable TV cable. First, the transmission characteristics of the downstream band differs from that of the upstream band. The upstream band (low band) is much noisier than the downstream band. To achieve the same data rate in both bands, the downstream band is made to conform to the upstream band. Thus, the method fails to achieve the full data rate potential of the downstream band.
Second, in most cases the upstream data is intended for a remote server and not for a peer end-user. Therefore, retransmitting the data on the downstream band wastes downstream band width.
Therefore it would be desirable to provide a high-speed data communication system, with a high efficiency rate, that utilize a shared cable, and that fully utilizes the available bandwidth of the transmission cable.
Other advantages and features of the invention will be made apparent in connection with the following detailed description.